A Magnetic Flux Leakage Method (MFL), which measures quantity of decrease in the thickness of the steel material by adding a magnetic field to the steel material and detecting a change of magnetic flux leakage, has been conventionally used as one of the methods to quantitatively evaluate the progress of corrosion in steel material (magnetic material) which are used for pipelines of oil or gas, a bottom plate of a crude oil tank, and the like. In such a Magnetic Flux Leakage Method, a hall element is generally used as a sensor for detecting magnetic flux leakage.
For example, in Japanese Unexamined Patent Application, First Publication No. 2005-3405, a technology to detect a ruptured portion of reinforcing steel covered by concrete, which is a non magnetic material, is disclosed. This technology detects a remaining magnetism of the reinforcing steel with a magnetic sensor and detects a portion where a large amount of magnetic flux leakage is generated as a ruptured portion.